Gonadotropin-releasing hormone (GnRH) neurons are central to the initiation and maintenance of reproductive function in diverse vertebrates. During development, GnRH neurons enter sequential stages to mature into a functional network capable of supporting reproduction in adulthood. These stages include cell fate specification in the olfactory placodes, migration into the forebrain, and targeting of axons to the median eminence for hormone release. Signals that trigger GnRH neuronal entrance into these stages, as well as factors that regulate maturation within each stage remain largely unknown. We hypothesize that neurotrophic factors provide time-specific signals to drive the progression of GnRH neurons development. In this proposal, we will use a candidate neurotrophic factor approach and focus on the actions of a family of growth factors shown to have profound neurotrophic activities in cultured primary and immortalized GnRH neurons: the fibroblast growth factors (FGFs). Further, the actions of insulin-like growth factor I (IGF-I), a neurotrophic factor previously shown to act independently and/or collaboratively with FGF-2, will be investigated. This proposal will utilize a combination of existing transgenic models and primary GnRH neuron cultures to address the following Aims: to determine 1) if receptors for FGFs are expressed in a time-specific manner during development, 2) if FGFs alter GnRH progenitor cell expansion and the emergence of GnRH neurons, 3) if FGFs alter the migration of GnRH neurons into the forebrain, 4) if FGFs promote GnRH axon targeting, and 5) if IGF-I acts independently or synergistically with FGFs to promote the survival of GnRH neurons. Together, results from these Aims will provide important clues regarding how a neuroendocrine system critical for vertebrate reproduction develops and matures with the guidance of neurotrophic factors. Further, these results will aid in the understanding of cellular and molecular basis of developmental reproductive abnormalities that result from GnRH deficiency. [unreadable] [unreadable]